The present invention relates to improved polyimide composites and methods of utilizing same for composite structural application. Specifically, the present invention is an improvement in that disclosed in U.S. Pat. No. 4,166,170 issued Aug. 28, 1979 and incorporated herein by reference.
Most addition type polyimide systems suffer from being a dry, powdery solid at room temperature. This requires these systems be dissolved in a solvent before they can be applied to glass or graphite to form an impregnated fiber or fabric called "prepreg" (pre-impregnated). Once the polyimide resin system has been made into prepreg, some of the solvent has to be retained in order to cause these sheets of prepreg to stick together and be flexible enough to work into complex shapes, for example, the fabrication of structural components for aircraft. If this solvent is lost due to volatility the result is a dry and brittle prepreg that is nearly impossible to work. The use of a solvent for this application, however, leads to another major problem--the removal of this liquid before the component is subjected to a 500.degree.-700.degree. F. cure (the reactive end groups cure through a vinyl-type polymerization at these temperatures). If all of the solvent is not removed, it will create void-areas in the cured part. These void-areas always cause the strengths of the resulting parts to be low.
All of the solvents that have been used with additiontype polyimides in the past are of the type that generate these volatiles during the cure of the resin. In the present invention, an approach is presented which eliminates this solvent problem.
It is therefore an object of the present invention to provide an addition type polyimide having improved physical property characteristics of tack and drape, i.e., the ability to stick and conform to complex shapes.
An additional object of the present invention is to provide an addition type polyimide resin system for providing laminates having excellent strength retention and improved thermooxidative stability.
A further object of the present invention is to provide an inexpensive polyimide resin system that is easy to fabricate and uses a high boiling point reacting solvent.
Another object of the present invention is a process for increasing the shelf life of an improved prepreg for use in fabricating composite structures.
Another object of the present invention is a novel process of preparing prepreg having enhanced tackiness property characteristics for use in fabricating composite structures.